Electrical resonance change in a wind turbine

ABSTRACT

A wind turbine including a plurality of elements including a tower, a nacelle mounted to the tower and a plurality of blades rotatable mounted to the nacelle is provided. At least one element of the tower, the nacelle and the blades includes a coaxial impedance member coaxially arranged about an axis of the element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No.PCT/EP2019/066698, having a filing date of Jun. 24, 2019, which is basedon EP Application No. 18181459.1, having a filing date of Jul. 3, 2018,the entire contents both of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to the field of wind turbines. A conventional windturbine comprises a plurality of elements including a tower, a nacellemounted to the tower and a plurality of blades rotatable mounted to thenacelle.

BACKGROUND

Such a conventional wind turbine has an electric resonance frequencywhich is normally fixed due to the size of the enclosure. Radiated radiofrequencies depend on how good the wind turbine emits the RF energygenerated mainly by the energy conversion components. Limits are set ininternational standards such as in the requirements for radiatedemissions for wind turbines. Here, some newly defined limit requirementsare defined down to a frequency of about 150 kHz. However, wind turbineshave a size corresponding to a few hundred kHz and are included in arange set by the recent standards.

Usually, emissions are reduced by implementing filters and by carefullyusing EMC workmanship like shielding. However, due to the requirementsfor radiated emissions, it becomes very difficult and expensive tosatisfy these limits as even a low current of some mA running in thesurface of the wind turbine would exceed the limits. Normally, thedimension of the element, i.e. the dimension of the enclosure of theelement defines the antenna characteristics of the element, but thisdimension cannot readily be changed.

There may be a need for a wind turbine which satisfies the EMC standardsunder low costs without burdensome redesigns of the wind turbine.

SUMMARY

According to a first aspect of embodiments of the invention, a windturbine comprises a plurality of elements including a tower, a nacellemounted to the tower and a plurality of blades rotatable mounted to thenacelle, wherein at least one element of the tower, the nacelle and theblades comprises a coaxial impedance member coaxially arranged about anaxis of the element. The axis of the element can be the longitudinalaxis thereof, which is the axis of the longest extension. The element isconfigured to exhibit electromagnetic oscillation along the axis with aresonance frequency of an order n, wherein the coaxial impedance memberis tuned to the resonance frequency of the order n.

Advantageously, the coaxial impedance member can function as a chokesuch that an electric current oscillating with the resonance frequencywithin the element is attenuated and the energy thereof is converted toheat in the coaxial impedance member. In addition or alternatively, thecoaxial impedance member can function as a detuning member such that theresonance frequency is split in two resulting resonance frequenciesbeing higher than the resonance frequency. With it, the EMC standardscan readily be satisfied under low costs.

The coaxial impedance member is configured to be placed at differentpositions along the axis of the element so that the choking and detuningeffects can be optimized.

The coaxial impedance member has a length substantially equal toapproximately one-quarter wavelength of the resonance frequency. Inparticular, the coaxial impedance member can thus be tuned to the firstorder resonance frequency which is thereby well absorbed.

The coaxial impedance member is formed of a tape or a sleeve comprisingelectrically conductive material. More preferred, the electricallyconductive material comprises a nanocrystalline or ferrite material. Thecoaxial impedance member allows detuning the resonance frequency in asimple and cost effective manner. Due to the very low common modecurrent in the tower surface, only a very small amount of tape materialis needed. The arrangement of such a coaxial impedance member canreadily be integrated in the manufacturing process of the tower or theother elements of the wind turbine, and it is even possible to retrofitexisting wind turbines by adding the coaxial impedance member.

In a second aspect of embodiments of the present invention, a tower fora wind turbine comprises a coaxial impedance member coaxially arrangedabout a longitudinal axis of the tower.

In a third aspect of embodiments of the present invention, a blade for awind turbine comprises a coaxial impedance member coaxially arrangedabout a longitudinal axis of the blade.

In a fourth aspect of embodiments of the present invention, a method ofsuppressing electromagnetic radiation having a resonance frequency of anorder n in a wind turbine comprising a plurality of elements including atower, a nacelle mounted to the tower and a plurality of bladesrotatable mounted to the nacelle, the method comprises a step ofarranging a coaxial impedance member coaxially about an axis of at leastone element of the tower, the nacelle and the blades.

The step of arranging the coaxial impedance member comprises a step ofplacing the coaxial impedance member at a position along the axis of theat least one element, where a current distribution or current density inthe element exhibits a maximum.

Arranging the coaxial impedance member comprises a step of wrapping atape of an electrically conductive material about the at least oneelement.

It has to be noted that embodiments of the invention have been describedwith reference to different subject matters. In particular, someembodiments have been described with reference to apparatus type claimswhereas other embodiments have been described with reference to methodtype claims. However, a person skilled in the art will gather from theabove and the following description that, unless other notified, inaddition to any combination of features belonging to one type of subjectmatter also any combination between features relating to differentsubject matters, in particular between features of the apparatus typeclaims and features of the method type claims is considered as to bedisclosed with this application.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with reference tothe following figures, wherein like designations denote like members,wherein:

FIG. 1 shows a wind turbine and the different elements thereof;

FIG. 2 shows a distribution of an electric RF-current in a wind turbine;

FIG. 3 shows electromagnetic oscillation with a first order resonancefrequency f₁ along the axis of the tower;

FIG. 4 shows electromagnetic oscillation with resulting resonancefrequencies along the axis of the tower equipped with a coaxialimpedance member;

FIG. 5 shows an embodiment of a tower comprising a plurality of coaxialimpedance members; and

FIG. 6 shows an embodiment of a tower comprising a coaxial impedancemember made of nanocrystalline material.

DETAILED DESCRIPTION

FIG. 1 shows a wind turbine 1. The wind turbine 1 comprises a nacelle 3and a tower 2. The nacelle 3 is mounted at the top of the tower 2. Thenacelle 3 is mounted rotatable with regard to the tower 2 by a yawbearing. The axis of rotation of the nacelle 3 with regard to the tower2 is referred to as the yaw axis.

The wind turbine 1 also comprises a hub 4 with three rotor blades 6 (ofwhich two rotor blades 6 are depicted in FIG. 1 ). The hub 4 is mountedrotatable with regard to the nacelle 3 by a main bearing 7. The hub 4 ismounted rotatable about a rotor axis of rotation 8.

The wind turbine 1 furthermore comprises a generator 5. The generator 5in turn comprises a rotor 10 connecting the generator 5 with the hub 4.The hub 4 is connected directly to the generator 5, thus the windturbine 1 is referred to as a gearless, direct-driven wind turbine. Sucha generator 5 is referred as direct drive generator 5. As analternative, the hub 4 may also be connected to the generator 5 via agear box. This type of wind turbine 1 is referred to as a geared windturbine. Embodiments of the present invention are suitable for bothtypes of wind turbines 1.

The generator 5 is accommodated within the nacelle 3. The generator 5 isarranged and prepared for converting the rotational energy from the hub4 into electrical energy in the shape of an AC power. Thereby, thegenerator 5 generates noise in the shape of RF radiation.

Each element of the wind turbine 1 has a certain dimension. The tower 2has a height A. The blades 6 form together a diameter C, and each blade6 has a length E. The nacelle 3 has a height B and a length D.

The wind turbine 1, especially the tower 2 thereof, is a largeconductive structure for RF-energy. The wind turbine 1 can be consideredas an antenna (particularly a monopole antenna). The physical dimensionof length determines the electromagnetic resonance frequency. For eachdimension A, B, C, D and E, a specific resonance frequency f_(n) of anorder n is present. Also each combination of any elements exhibits aspecific resonance frequency f_(n) of an order n. FIG. 2 shows anexample of a distribution of an electric RF-current in the wind turbine1.

For the first order harmonics (n=1), the quarter wavelength ¼λ₁ is themost effective length for an antenna, where resonance can be used fortransmitting or receiving radio signals. FIG. 3 shows a first orderharmonics within the tower 2 along the longitudinal axis of the tower 2.The length 1 of the element corresponds to ¼λ₁ of the resonancefrequency f₁ of the first order harmonics (n=1).

According to embodiments of the present invention, a coaxial impedancemember 10 is coaxially arranged about a longitudinal axis of the tower 2and axially aligned with the tower 2. The term “coaxially arranged abouta longitudinal axis” includes a case where the coaxial impedance member10 is arranged coaxially to and radially outside of the tower 2, and acase where the coaxial impedance member 10 is arranged coaxially to andembedded in the tower 2. The term “longitudinal axis” normally refers tothe axis of the longest extension of the tower 2, i.e. in the uprightdirection of the tower 2. The coaxial impedance member 10 can bearranged onto a circumferential surface of the tower 2.

The same applies to embodiments where the coaxial impedance member 10 isarranged at another element such as the nacelle 3 and the blades 6. Withregard to the nacelle 3 as shown in FIG. 1 , one coaxial impedancemember 10 can be coaxially arranged about the vertical axis, and anothercoaxial impedance member 10 can be arranged about the horizontal axis,i.e. the rotational axis 8.

The coaxial impedance member 10 has an inductance. The impedance memberis tuned to the resonance frequency f₁ of the order 1. The coaxialimpedance member 10 may have a length which is substantially equal toapproximately one-quarter wavelength λ₁ (¼λ₁) of the resonance frequencyf₁ of the first order harmonics (n=1). By arranging the coaxialimpedance member 10, two advantageous effects are obtained:

First, the coaxial impedance member 10 acts like choke so that theresonance frequency f₁ is choked. That means, the RF-current in thetower 2 is attenuated and converted to heat in the coaxial impedancemember 10. The energy of the resonance frequency f₁ of the order n=1 isabsorbed by the impedance member 10. The coaxial impedance member 10 isplaced to a position, where a current distribution in the tower 2exhibits a maximum so that a large amount of energy is absorbed.

Second, the resonance frequency f₁ of the order 1 is detuned. FIG. 4shows that the physical length 1 (corresponding to the length A in FIG.1 ) of the tower 2 is split in two parts by the coaxial impedance member10. The resulting resonance frequencies are higher than the originalresonance frequency. For example, if the coaxial impedance member 10 isarranged in the middle of the length A, the original resonance frequencyf₁ having a wavelength ¼λ₁=A is split in two resonance frequenciesf_(1a) and f_(1b), each having a resulting wavelength¼λ_(1a)=¼λ_(1b)=A/2. In other words, the coaxial impedance member 10moves up (increases) the resonance frequency. The coaxial impedancemember 10 functions as a detuning member. By detuning the resonancefrequency, the amount of energy is potentially further reduced.

If the coaxial impedance member 10 is placed anywhere to the tower 2except for the axial center, a first one of the two resulting resonancefrequencies is higher than the original resonance frequency, and asecond one of the two resulting resonance frequencies is not only muchhigher than the original resonance frequency but also higher than thefirst one of the two resulting resonance frequencies.

FIG. 5 shows an embodiment, where the tower 2 is provided with aplurality (three) of coaxial impedance members 10 so that the originalresonance frequency is split a plurality of times (three times). In theembodiment described so far, the coaxial impedance members 10 arearranged on the tower 2. However, it is clear that a coaxial impedancemember 10 can be arranged in addition or alternatively at any otherelement of the wind turbine 1 such as the nacelle 3 and/or one or moreblades 6, for example on a circumferential surface thereof.

FIG. 6 shows that the coaxial impedance member 10 is formed of a tape ofelectrically conductive material and is arranged about and axiallyaligned with the tower 2. Due to the very low common mode current in thetower surface, only a very small amount of tape material is needed. Thetape comprises an electrically conductive material which can include ananocrystalline or ferrite material. The nanocrystalline material can bea nanocrystalline alloy. The electrically conductive material can beapplied onto a substrate member such as a flexible sheet. The sheet canbe made of a synthetic resin such as polyester or metal such asaluminum.

Next, a method of suppressing an electromagnetic radiation in a windturbine 1 is described. The coaxial impedance member 10 is configured tobe placed at different axial positions of the tower 2. The coaxialimpedance member 10 is placed to a position, where a currentdistribution in the tower 2 exhibits a maximum. The maximum currentdistribution in the tower 2 can be found by measurements or can bedetermined based on empiric data. Generally, the axial position of theelement means a position along a longitudinal axis of the element. Thelongitudinal axis of the element is normally the axis of the longestextension of the element.

If the coaxial impedance member 10 is formed of a tape, the same iswrapped or wound about the tower 2 at the determined position.

According to embodiments of the present invention, the wind turbine 1can satisfy the requirements for radiated emissions which define limitsin the frequency range down to 150 kHz. The coaxial impedance member 10allows detuning the resonance frequency in a simple and cost effectivemanner. The arrangement of the coaxial impedance member 10 can readilybe integrated in the manufacturing process of the tower 2 or the otherelements of the wind turbine 1. It is even possible to retrofit existingwind turbines 1 by adding the coaxial impedance member 10 ex post.

Furthermore, a choking effect can be achieved, where RF-energy isconverted to heat, particularly when the coaxial impedance member 10 isarranged at a location of a high or the highest current density. Forradiation emissions, a relative small amount of material is needed forthe coaxial impedance member 10 due to very low currents.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of “a” or“an” throughout this application does not exclude a plurality, and“comprising” does not exclude other steps or elements.

The invention claimed is:
 1. A wind turbine comprising: a plurality ofelements including a tower, a nacelle mounted to the tower and aplurality of blades rotatable mounted to the nacelle, wherein at leastone element of the tower, the nacelle, and the plurality of blades isconfigured to exhibit electromagnetic oscillation along an axis of theat least one element with a resonance frequency (fn) of an order n, andcomprises a coaxial impedance member that is coaxially arranged aboutthe axis of the at least one element, and is tuned to the resonancefrequency of the order n.
 2. The wind turbine according to according toclaim 1, wherein the coaxial impedance member functions as a choke suchthat an electric current oscillating with the resonance frequency (fn)within the at least one element is attenuated and the energy thereof isconverted to heat in the coaxial impedance members; and/or the coaxialimpedance member functions as a detuning member such that the resonancefrequency (fn) is split in two resulting resonance frequencies (f_(1a)and f_(1b)) being higher than the resonance frequency (fn).
 3. The windturbine according to claim 1, wherein the coaxial impedance member isconfigured to be placed at different positions along the axis of the atleast one element.
 4. The wind turbine according to claim 1, wherein thecoaxial impedance member has a length substantially equal toapproximately one-quarter wavelength.
 5. The wind turbine according toclaim 1, wherein the coaxial impedance member is formed of a tape or asleeve comprising electrically conductive material.
 6. The wind turbineaccording to claim 1, wherein the electrically conductive materialcomprises a nanocrystalline or ferrite material.
 7. A tower for a windturbine, comprising a coaxial impedance member coaxially arranged abouta longitudinal axis of the tower; wherein the tower is configured toexhibit electromagnetic oscillation along an axis of the tower with aresonance frequency (fn) of an order n, and wherein the coaxialimpedance member is tuned to the resonance frequency of the order n. 8.A blade for a wind turbine, comprising a coaxial impedance membercoaxially arranged about a longitudinal axis of the blade; wherein theblade is configured to exhibit electromagnetic oscillation along an axisof the blade with a resonance frequency (fn) of an order n, and whereinthe coaxial impedance member is tuned to the resonance frequency of theorder n.
 9. A method of suppressing electromagnetic radiation having aresonance frequency (fn) of an order n in a wind turbine, the windturbine comprising a plurality of elements including a tower, a nacellemounted to the tower, and a plurality of blades rotatable mounted to thenacelle; wherein at least one element of the tower, the nacelle, and theplurality of blades is configured to exhibit electromagnetic oscillationalong an axis of the at least one element with a resonance frequency(fn) of an order n, and the method comprising arranging a coaxialimpedance member coaxially about an axis of at least one element of thetowers, the nacelle and the plurality of blades; wherein the coaxialimpedance member is tuned to the resonance frequency of the order n. 10.The method according to claim 9, wherein the step of arranging thecoaxial impedance member comprises placing the coaxial impedance memberat a position along the axis of the at least one element, where acurrent distribution or current density in the element exhibits amaximum.
 11. The method according to claim 9, wherein arranging thecoaxial impedance member comprises a step of wrapping a tape of anelectrically conductive material about the at least one element.